1. Field of the Invention
The present invention relates generally to the prediction of weather disturbances and, more particularly, to the prediction of weather disturbances that give rise to microburst wind shear conditions at low altitudes over the earth's surface which are hazardous to aircraft during takeoff and landing.
2. Description of the Prior Art
A microburst is a powerful downward blast of air, usually associated with a thunderstorm or rain, followed by wind shear, a violent horizontal burst of air in all directions at low altitudes. Wind shear, which is extremely hazardous during aircraft takeoffs and landings, occurs over a relatively small region and typically lasts 5 to 15 minutes.
A system for providing an early warning of wind shear conditions is disclosed by W. L. Rubin et al in U.S. patent application Ser. No. 07/683,356 entitled "Microburst Precursor Detection Utilizing Microwave Radar" and is assigned to the assignee of the present invention. In accordance with this prior art, early warning of wind shear conditions is provided by detecting the vertical wind downdraft during its descent before it reaches ground level and establishes the conditions that generate wind shear. Vertical wind downdraft is determined by extracting four weather parameters from received signals of a scanning single beam or vertically stacked multiple beam microwave doppler radar system which illuminates a preselected altitude range for a predetermined distance about an airport. The number of beams of the doppler radar system and their beamwidths are designed to provide coverage over the preselected altitude range in a manner that establishes a vertical and horizontal limit for each range cell of the doppler radar system for all slant ranges that are less than a predetermined distance. The horizontal limit is selected to insure that a vertical wind downdraft column completely fills the beam, while the vertical limit is selected to restrict the effects of wind velocity gradients within a range cell. The received radar signals are processed to establish mean radial velocity, spectral width and skewness of the precipitation doppler velocity spectrum, and precipitation reflectivity in each range-azimuth cell in an illuminated azimuth-elevation sector. This data is then utilized to establish hydrometeor (precipitation) vertical velocity, horizontal wind velocity, and spatial location and extent of these parameters. The vertical wind velocity, spatial extent, and reflectivity are then compared to meteorological characteristics of storm generated microburst precursors: a vertical wind downdraft velocity of at least five meters per second, a vertical wind downdraft column between 1.5 and 3.0 kilometers in diameters, and an increase in precipitation reflectivity of 0-20 dB over that of the surrounding regions within the vertical wind downdraft. All of these criteria are utilized to confirm that a microburst generating downdraft has been initiated.
The doppler velocity spectrum within an elevated antenna beam is established by combining the radial component of vertical rain drop velocity, which is a function of the sine of the beam elevation angle, with the radial component of horizontal rain drop velocity, which is a function of the cosine of the beam elevation angle, over the beamwidth of each elevated beam. This velocity spectrum is unique for each combination of average vertical and horizontal hydrometeor velocities within each range azimuth cell. The measured doppler spectrum parameters in each range-azimuth cell in each beam within the illuminated altitude region are stored on successive radar scans to establish a four dimensional map. Measured doppler spectral parameters include mean doppler velocity, doppler spectrum width, doppler spectrum asymmetry, and total doppler spectral power in the radar echo. These measured parameters of hydrometeors immersed in a microburst downdraft provide the basic information from which microburst precursor vertical and horizontal wind velocity can be estimated. When it is determined from these maps that a vertical wind column of between 1.5 and 3.0 kilometers diameter, having a vertical wind velocity which exceeds five meters per seconds and exhibits a precipitation reflectivity that is 0-20 dB above the surrounding areas has been detected, a microburst warning is generated. Since time for the vertical downdraft to descend to the earth's surface from the maps data area is in the order of five minutes, this warning will precede the occurrence of subsequent surface microburst wind shear by a time that is adequate to divert or to delay an aircraft takeoff or landing.
The aforementioned patent application includes a microburst downdraft verification mode, wherein a paired set of measurements of (.beta., V.sub.RAD) in each range-azimuth cell, V.sub.RAD being the mean radial doppler velocity and .beta. the mean doppler spectral skewness, are processed to provide a paired estimate of (V.sub.V, V.sub.H), V.sub.V being the mean vertical velocity, V.sub.H the mean horizontal velocity of raindrops in the range-azimuth cell, and a paired set of measurements (.sigma., V.sub.RAD), .sigma. being the mean doppler spectral width, are processed to provide a complementary paired estimate (V.sub.V, V.sub.H) in the range-azimuth cell.
Two difficulties exist in this prior art system. First, since the magnitude of the spectral skewness .beta. is small, an accurate estimate of this parameter requires a very large number of data samples (received radar pulses for processing). Second, to obtain the paired set (V.sub.V, V.sub.H) from the paired set (.sigma., V.sub.RAD) the system assumes that the raindrop turbulence is 1.0 meter per second. Actual raindrop turbulence deviating from this assumed value produce errors in the paired set (V.sub.V, V.sub.H). Though meteorological data indicates that raindrop turbulence is generally in the order of 1.0 meter per second, it is desirable to eliminate the dependence upon this assumption and provide a more accurate paired set (V.sub.V, V.sub.H) when the raindrop turbulence differs from this value.